Hello need help...

gcds

Member
2007-03-09 10:19 pm
Hello guys i building some system i need preamp with volume control based on pga2311... I have made some schematics but i wanted that you pro's could check it and tweak it :D

Inputs will be:
1. iPod, iPhone, Walkman.... 3.5mm jack (MP3)
2. China USB Audio card
3. Bluetooth Headsets like mp3 i think so
4. iPod Dock connector line out (which is not like headphone jack..)
5. FM module based on AR1010

Output Will be TC2000 and TP2050 based amplifier with spec like:
Frequency Response: 20HZ—20KHz(±3dB)
Input Sensitivity: Min. 0.720V; Typ. 0.771V; Max. 0.800V
Channel Separation: Min. 80dB; Typ. 95dB
Input Impedance: Min. 20K ohms; Typ. 22K ohm

This will be Portable speakers and need to get maximum of amplifier but not like you even cant understand words of song :DDD

EDIT: IC is opa134

Regards
Aurimas
 

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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
It looks OK... my only real reservation is limited headroom due to -/+5 volt rails. The output from a CD player... which is one source you haven't mentioned... is typically 2 volts rms at max output which is nearly 6 volts pk/pk. Feed that into an amp with a gain of 2 running on 10 volts dc and it will clip.

The 0.1 uf's could usefully be increased as could R2 and R3 to extend the LF turnover point... unless you deliberately don't want extended LF response.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
I have looked at the data sheet for for PGA2311 and that places absolute limits on the audio that can be applied.

The outputs from the sources you mention will all be less than a CD player I imagine, but maybe not much less. I would not fit R5 and R6 which will make the opamp act as a buffer with a gain of 1. R7 and R9 can either be left fitted... which gives the option of re-fitting R5 and 6 if you ever did need more gain... or can be linked out.

Also the data sheet recommends a low <600 ohm source impedance so I would drop R8 and R10 right down to around 47 ohms which will give a stability margin for the opamp if there is any stray capacitance that it works into.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
OPA134 is fine... remember to add a 0.1uf cap directly across it's supply pins to ensure stability.

The gain... only you can decide that. Try it without R6 to begin with. If you need more gain then fit R6 and select it's value on test. I think it will be OK without.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Initially leave it out all together... that gives a gain of 1.

If you need more gain... and I dont think you will... then fit a 10k as you show. That will give a gain of 2.
Or you can fit a preset resistor of 10k in place of R6 and adjust until the gain is OK, then take it out, measure it and fit the nearest fixed value resistor.